A bonding process, a coating process, and a shielding process are important in a material processing and a material molding, and a variety of materials are used in the processes. Materials that are capable of being added or coated at room temperature and have a property of being cured by some subsequent action have been used through the ages especially from the viewpoint of workability.
In general, many of adhesive agents or coating materials are used in a method in which a pigment or a resin is dissolved in an organic solvent or water, an intended mixture is coated with the pigment or the resin in the form of liquid, the organic solvent or the water is volatilized, and thus a cured product is obtained. However, there are problems in that when the organic solvent is used, a health hazard or a possibility of catching fire may be caused by gas that is volatilized, and in that when the water is used, it takes time to obtain the cured product due to low volatilization of the water. In order to solve these problems, the following methods (1) to (4) are mainly used as a method in which substances in the form of liquid are cured after addition and coating without using a solvent.
(1) A method in which two low-volatile liquid compounds that react to each other are cured by being blended and immediately added and coated to accelerate the reaction.
(2) A method in which a low-volatile liquid compound is cured by reaction to water in the air after addition and coating of the compound.
(3) A method in which a low-volatile liquid compound is cured by initiating a curing reaction by heating the compound after addition and coating of the compound.
(4) A method in which a low-volatile liquid compound is cured by causing a reaction by irradiating the compound with light or an electron ray after addition and coating of the compound.
In the method (1), a reaction of an epoxy group is used in the curing reaction, and a curing agent such as polyamine and polyol is blended into an epoxy compound such as glycidyl bisphenol A. An epoxy compound is widely used in the curing reaction because the cured epoxy resin has high mechanical strength, and is excellent in many characteristics such as electric characteristics, heat resistance, water resistance, and chemical resistance.
However, the two-liquid blended material of the method (1) is remarkably low in preservation stability after the blending, so that the blended material needs to be used immediately. Thus, the two compounds need to be blended at every use.
In the method (2), a reaction of an alkoxy silyl group such as a room-temperature moisture-curing modified silicone resin, a reaction of an α-cyanoacrylate group such as a cyanoacrylate adhesive, and a reaction of an isocyanate group such as an isocyanate group-containing urethane polymer are used in the curing reaction, and blending or heating is not required in the reaction. However, the curable compositions have the following shortcomings.
The room-temperature moisture-curing modified silicone resin and the isocyanate group-containing urethane polymer have a shortcoming that curing the compounds takes a few tens of minutes to a few tens of hours, and further, if the compounds are coated a little thicker, the inside is not solidified while only the surface is solidified. In addition, the cyanoacrylate adhesive has a shortcoming that while the curing reaction is initiated with absorbed water on an adherend surface, the reaction to water in the air is slow, and curing the entire adhesive takes a few minutes to a few hours when the adhesive is used in molding or coating where the adhesive is not sandwiched between a pair of adherends, and further, curing the adhesive takes more time when internal curing is performed.
In order to correct the shortcomings regarding the curing speed or the internal curing described above, attempts to add a basic compound or an organic metal as a catalyst to the compounds have been made. However, there is a conflicting problem in that when the reactivity of the compounds to water is increased, the preservation stability of the compounds is lowered on the other hand.
Also in the method (3), a reaction of an epoxy group is used in the curing reaction, and a curing reaction is caused as in the method (1) while making the curing agent inactivate at room temperature by increasing the melting point of the curing agent, introducing a protective group having a thermal dissociation property into the curing agent, or microencapsulating the curing agent. The curing agent is blended, and heated, whereby the curing agent is activated to cause a curing reaction. This method requires no blending, and is capable of uniformly curing only the portion of the compound to which heat is applied in a relatively short time.
However, the curing reaction of the method (3) requires heating in order to cure the compound, so that when a thick cured product is to be obtained, or the compound is cured while an adherend is cured together, a long heating time or a special heating method is required in order to make the heat spread the entire compound to be raised up to a curing initiation temperature. In addition, there is a problem in that the compound needs to be refrigerated because the curing proceeds slowly at room temperature.
In the method (4), a radical polymerization reaction of a compound in which molecules have double bonds such as an acrylate derivative is used in the curing reaction. That is, a compound that generates radicals by being irradiated with light or an electron ray is blended into an acrylate derivative in advance, and a curing reaction is caused by irradiating the compound with light or an electron ray in the method (4). Using the compound that is a radical species having high activity, this method requires no blending or heating, and is capable of curing the compound in a short time.
However, in the method (4), while having high activity, the radical species have a short lifetime and easily have their activity lost by oxygen, so that as soon as the irradiation is stopped, the curing reaction stops. Thus, there is a problem in that the portion of the compound that ultraviolet irradiation light does not reach (dark portion) cannot be cured.
In order to solve these problems, methods described in Patent document 1 to Patent document 5 are proposed. However, these methods have the following problems.
Described in Patent document 1 is a method that ultraviolet curing and moisture curing are used in combination, where the compound is moisture cured to a dark portion after ultraviolet curing. However, the moisture curing takes time, and curing the entire compound takes a same time as curing the moisture-curing resin.
Described in Patent document 2 is a method that an epoxy resin is cured by a photo cation polymerization initiator that generates a cation by being irradiated with ultraviolet light. The cations (Lewis acid) generated in this method have a long lifetime unlike the radicals; however, there is a problem in that the cations pollute an adherend such as a metallic material because the cations are acid. In addition, a cation generating agent is generally a special kind of ionic pair and high in cost.
Described in Patent document 3 is a method that curing is initiated by insulating oxygen. A dark portion of a bonded surface where oxygen is intercepted can be cured effectively; however, a thick compound or an open type compound cannot be cured. That is, this method cannot be used for coating or sealing.
Described in Patent document 4 is a method that curing is initiated with near infrared light. The near infrared light, which is transmitted at a high rate through a substance, is capable of curing a portion of a compound that ultraviolet light or visible light cannot reach; however, it is difficult for the near infrared light to obtain high energy, so that curing the compound takes a few minutes. In addition, an irradiation device that is higher in cost than a UV light irradiation device is required in most cases.
Described in Patent document 5 is a method that an ultraviolet radical generating agent and a thermal radical generating agent are used in combination. A dark portion of a compound can be cured by radicals that are generated from the thermal radical generating agent; however, the thermal radical generating agent needs to be stored by using a special kind of deactivation method because a peroxide derivative is used as the thermal radical generating agent, so that a reaction does not occur if preservation stability of the thermal radical generating agent is improved. In addition, the peroxide derivative has a property of oxidation decomposing an organic substance even at room temperature, so that preservation stability of the thermal radical generating agent is remarkably lowered even with a slight rise in temperature. In addition, when the peroxide derivative remains after the curing, the peroxide derivative causes deterioration of the cured product.
In addition, the radical polymerization has a shortcoming such that if the generated radicals are partially uneven in amount, the polymerized material is accordingly uneven in molecular mass, so that a highly-reliable material cannot be obtained.
In order to solve these problems, a mechanism to capture and transport generated radicals to a site where radicals are small in amount or a site where radicals are not generated is studied as a method for curing a portion of a compound where the radicals are small in amount such as a dark portion without using a thermal radical generating agent such as a peroxide derivative. A chain transfer agent is considered as the mechanism.
A chain transfer agent has a special kind of structure, and is capable of capturing or transporting radicals. A chemical structure of a compound in which molecules have atoms of sulfur such as a thiol compound, and a chemical structure of a compound having an α-methylstyrene dimer, a methacrylic ester n-mer, and aromatic atoms of nitrogen of imidazole compound are studied as the chemical structure of the chain transfer agent. Those compounds are descried in the following Patent document 6 to Patent document 9, and Non-patent document 1 to Non-patent document 4.
In addition, as ultraviolet curable materials expand in application, the ultraviolet curable materials have large problems relating to improvement in physical property and functionalization of cured products in addition to a curing property. In order to solve these problems, materials prepared by instilling or blending a variety of materials in the ultraviolet curable materials have been reported (see Patent document 10 to Patent document 14).
Patent document 10 describes instilling a powdered organic compound and a fibrous organic compound in an ultraviolet curable material in order to improve the thermal expansibility and the smoothing property of the ultraviolet curable material.
Patent document 11 describes instilling a conductive metallic oxide, a metallic powder, conductive carbon black, alkali metal salt of lithium compound, or quaternary ammonium salt in an ultraviolet curable material in order to prevent static charge of the ultraviolet curable material.
Patent document 12 describes blending a polymer in an ultraviolet curable material in order to improve the light resistance of the ultraviolet curable material.
Patent document 13 describes instilling a light-shielding filler or a filler for light attenuation in an ultraviolet curable material in order to improve the accuracy of the form of the ultraviolet curable material.
Patent document 14 describes instilling carbon, carbon fibers or carbon cloth in an ultraviolet curable material in order to improve the strength of the ultraviolet curable material.
As described above, the methods for instilling the variety of fillers in the ultraviolet curable materials are described in Patent documents 10, 11, 13 and 14, and the method for blending the polymer in the ultraviolet curable material is described in Patent document 12. Especially if the latter polymer can be blended in the ultraviolet curable material without any limitation, the form of the curing materials before the curing is not limited to the liquid form, and curing materials in the forms of a wax, a sheet, and a tape can be produced because the physical properties of the curing materials can be easily changed. Thus, the curing materials in the forms of a wax, a sheet, and a tape can find a wide variety of applications as new materials.
However, when infilling or blending the solid materials and the polymer in the ultraviolet curable materials, most of the solid materials and the polymer shield, absorb, or scatter the projected ultraviolet light, and prevent the projected ultraviolet light from being transmitted through themselves (i.e., most of the solid materials and the polymer define ultraviolet transmission inhibitors). For this reason, the ultraviolet light does not reach deep portions of the ultraviolet curable materials sufficiently, and the deep portions are not cured sufficiently.
This insufficient curing results from the properties of curing reaction type radicals that are generated by the ultraviolet light irradiation. The properties are based on the curing reaction principle of the ultraviolet curable materials, that is, the radicals have properties such that the generated radicals have a remarkably short lifetime to easily have their activity lost by oxygen, so that polymerization reaction does not occur outside of the region where the radicals are generated.
In order to solve these problems, a variety of methods in order to infill or blend the substances in the ultraviolet curable materials are devised in the above-described literatures. Examples thereof include method (A) to method (E) as follows.
Method (A): reported in Patent document 10 is a method for reducing the average particle diameter of the powdered organic compound to the ultraviolet wavelength or less, a method for not preventing the ultraviolet transmission by using glass fiber capable of transmitting light as the fibrous organic compound.
Method (B): reported in Patent document 11 is a method for not preventing the ultraviolet transmission by enhancing the efficiency of the infilling by improving the physical properties of the resin to reduce the used amount of the infilling to 5% or less.
Method (C): reported in Patent document 12 is a method for not preventing the ultraviolet transmission by making two layers of the blended polymer and the ultraviolet curable material and limiting the optical absorption wavelength of the blended polymer to a wavelength that does not prevent the ultraviolet curing.
Method (D): reported in Patent document 13 and Patent document 14 is a method for curing an epoxy resin of a matrix by generating cations that have a relatively long lifetime by the ultraviolet light irradiation and dispersing the cations to the back side of a shielding attenuating filler.
Method (E): also reported in addition to the above-described methods is a method for curing a portion of the material where the ultraviolet light does not reach while there is no description of a curing property of a material that contains a ultraviolet transmission inhibitor (see Patent document 1 and Patent document 15). Above-descried Patent document 1 describes a method for moisture curing a portion of the material where the ultraviolet light does not reach in combination with curing a silicone resin. Above-descried Patent document 15 describes a method for anionically-polymerizing by water a portion of the material where the ultraviolet light does not reach while using 2-cyanoacrylate.